Fluidized catalytic cracking (FCC) is well known and widely used for conversion of heavier feeds boiling in the gas oil and heavier range to lighter products including gasoline.
One of the problems encountered in FCC processing is that the heavy feeds processed contain metals, such as nickel and vanadium, which deposit on the circulating FCC catalyst. The deposited Ni+V act as catalyst poisons, promoting undesirable coke formation as well as excessive hydrogen and light gas formation.
Refiners have resorted to several tactics to avoid nickel and vanadium poisoning during FCC processing. The approaches could be arbitrarily classified three ways:
1. Keep Ni+V out of the FCC feed; PA1 2. Leave Ni+V in the feed, but passivate or trap the metals once they reach the FCC catalyst; and PA1 3. Allow Ni+V to deposit on the catalyst and use a magnetic separation process to remove the oldest, most metals-contaminated catalyst.
Approach no. 1, keeping the Ni+V out of the feed, has been used for over fifty years. The simplest and highly effective way to keep metals out is to distill the FCC feed. Distilled feeds are usually metals-free, or have such low metals levels that no special steps need to be taken to deal with Ni/V contamination. Distillation is simple, inexpensive, and widely used. One drawback is that distillation keeps significant amounts of potentially high value, readily convertible hydrocarbon out of the FCC unit. Phrased another way, if a refiner limits the feed to the FCC unit to distillable feeds, a lot of profit is left in the non-distillable, or residue fraction of the crude oil charged to the refinery. Solvent deasphalting of heavy feeds is effective at removing most metal contaminants from even non-distillable hydrocarbon feed. Deasphalted oil (DAO) can be charged to the FCC, with the asphalt fraction used for road construction, or sent to a coker. The drawback to this approach is the significant capital and the operating expense of operating a de-asphalting unit.
Most refiners are driven by economics to process some residual fractions in their FCC units and use approach no. 2, metals passivation. Thus, many FCC units now process feeds with a few weight percent resid up to 10 or 20 weight percent resid. With such feeds comes Ni and/or V catalyst contamination. One way to tolerate higher metals levels on FCC equilibrium catalyst is to passivate the deposited contaminating metals. Although many metal passivators are known, an especially effective and widely used passivator is antimony.
Metals passivation, usually by antimony addition, is probably the most popular method in the world for solving the problem of heavy metals in feeds. There are some drawbacks to use of Sb for metals passivation. Antimony is expensive, potentially toxic, and fugacious. It is difficult to run an accurate antimony balance around a typical FCC unit. Some of the antimony is believed to deposit on the walls of fired heaters, or perhaps on other solid surfaces within the FCC unit. There is enough problem with such deposits that U.S. Pat. No. 4,167,471 was granted on the discovery that adding the antimony compound after the FCC feed heater, rather than before, increased the amount of antimony that ended up on the catalyst.
Even with antimony injection after the FCC feed preheater, much of the antimony addition has been difficult to trace. Because of difficulties with Sb addition, refiners are now considering approach no. 3, magnetic beneficiation. The third method of dealing with excessive amounts of Ni and/or V in the FCC feed is to use the MagnaCat.RTM. magnetic catalyst separation process developed by Ashland Petroleum Company, Refining Process Services and the M.W. Kellogg Company. More details of this process are disclosed in one or more of the following patents:
U.S. Pat. No. 4,406,773 discloses magnetic separation of high activity catalyst from low activity catalyst.
U.S. Pat. No. 5,106,486 (Re. 35,046) teaches adding iron compound continuously or periodically to the circulating catalyst.
U.S. Pat. No. 5,147,527 covers the concept of using a magnetic rare earth roller device (RERMS) for magnetic separation.
U.S. Pat. No. 5,171,424 teaches the use of highly paramagnetic heavy rare earths as Magnetic Hook.TM. additives that increase catalyst performance.
U.S. Pat. No. 5,190,635 teaches accumulation of iron on the catalyst and formation of superparamagnetic or ferromagnetic species.
U.S. Pat. No. 5,230,869 covers the discovery of a highly superparamagnetic species, which when present in aged equilibrium catalyst, further improves separation due to its high magnetic susceptibility compared to normal paramagnetic iron.
U.S. Pat. No. 5,328,594 teaches use of heavy rare earths as Magnetic Hook.TM. additives.
U.S. Pat. No. 5,364,827 teaches adding amounts of magnetically active moieties, over time, so the moiety deposits on catalyst or sorbent in an FCC unit or similar circulating hydrocarbon conversion unit which can be separated from catalyst which has been in the system a shorter time.
U.S. Pat. No. 5,393,412 teaches a catalyst recovery unit ancillary to an FCC or similar unit, which permits magnetic separation, sieving and attriting of equilibrium catalyst.
U.S. Pat. No. 5,538,624 teaches retaining specialty additives by doping them with lots of magnetic metals.